Electromodulation of the surface-plasmon dispersion relation and adsorption of anions at the electrochemical interface

A new theory is elaborated for the surface-plasmon frequency SP(k?) dependence on the charge of the electrode. Its essential feature is to account for adsorption of anions or oxygen (primary oxidation processes) induced by anodic polarization, which modifies the free-electron distribution near the metal surface in contact with the electrolyte. It is shown that the bending of the electromodulation plot near the potential of zero charge [constancy of SP(k?) in the cathodic range and an almost linear decrease with anoidic polarization] is entirely determined by the charge-induced adsorption. The bending point does not depend on the intensity of induced adsorption (varied in the model simulation) but on the critical value of the charge at which the adsorption starts. The surface-plasmon electromodulation plots can thus be used for evaluation of this important quantity. Critical charges of adsorption so far obtained prove to be in good accord with evaluations obtained by independent studies (electromodulation of reflected phase, traditional adsorption and impedance measurements) in a series of metals and single-crystal faces.